Measuring thermal expansion and the thermal crown of rolls

ABSTRACT

Systems and methods for measuring the thermal crown of rolls in-situ (e.g., at high temperature) either inside or outside the rolling mill can include sensors that measure propagation times of mechanical waves through the rolls. In some embodiments, one or more sensors are used to measure the propagation times of ultrasonic waves traveling inside the roll and normal to the roll&#39;s axis. These measurements can be taken when the roll is still hot and can be used to determine in real-time the thermal expansion at various points along the roll.

REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of and claims the benefit of U.S. patent application Ser. No. 14/203,695 filed Mar. 11, 2014 which claims the benefit of U.S. Provisional Patent Application No. 61/776,925 filed Mar. 12, 2013, each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present application relates to systems and methods for measuring the thermal expansion and thermal crown of rolls in-situ.

BACKGROUND

Rolling is a metal forming process in which stock sheets or strips are passed through a pair of rolls to reduce the thickness of the stock sheets or strips. Due to the high temperatures generated from the friction of rolling, from material deformation, and/or from contacting hot incoming material, the rolls may experience thermal expansion (also referred to as thermal crown). Thermal expansion along the roll axis is referred to as the thermal crown and the average in thermal expansion along the roll axis is referred to as the thermal expansion. Accurate measurements of the thermal expansion/crown of the roll when the roll is hot are needed for many reasons, one of which is to ensure that proper adjustments are made when needed to position the rolls properly relative to the strips to ensure that the rolled metal strips are of the desired flatness and profile.

Because of the high roll temperatures and the environment of a mill, however, it is difficult to measure the profile/camber of the rolls at the required time during the rolling process. Numerical models are therefore used to simulate the evolution of the thermal expansion and thermal crown of the roll by estimating the initial conditions and the heat transfer at the roll surface. Although these numerical models do not require direct measurements, the results are limited in accuracy because of the difficulty of accurately estimating the model parameters. In some cases, thermal crown is inferred using flatness or profile measurements of the strip as it exits the roll bite, but such methods are of limited accuracy and are only useful if the entry profile of the sheet is known accurately, the mill is a single stand mill, and the mill is running. These methods also only apply to the portion of the roll in contact with the strip, and so the thermal crown of the roll located outside the strip must be estimated. In a similar way, the thermal expansion can be inferred using the measured exit strip thickness, but limitations similar to those associated with the inferred crown method also exist.

Other attempts at measuring the thermal crown of a roll involve measuring the distance between a sensor and a roll, which also has limitations. For instance, the beam upon which such sensors are mounted may deform, rendering the sensors inaccurate. Efforts to minimize beam deformation or compensate for beam deformation can be cumbersome (e.g., occupy a significant amount of space on/near the machinery) and expensive.

SUMMARY

The term embodiment and like terms are intended to refer broadly to all of the subject matter of this disclosure and the claims below. Statements containing these terms should be understood not to limit the subject matter described herein or to limit the meaning or scope of the claims below. Embodiments of the present disclosure covered herein are defined by the claims below, not this summary. This summary is a high-level overview of various aspects of the disclosure and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this disclosure, any or all drawings and each claim.

Systems and methods are disclosed for measuring the thermal expansion and/or thermal crown of rolls either inside or outside a rolling mill. In some embodiments, the measurement is obtained by measuring the change in propagation time of an ultrasound wave traveling inside the roll when the roll is at different temperatures. Some measurements are capable of being made while the rolls are at high temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

The specification makes reference to the following appended figures, in which use of like reference numerals in different figures is intended to illustrate like or analogous components.

FIG. 1 is an isometric view of a roll including a sensor.

FIG. 2 is a schematic end view of a roll positioned relative to a sensor.

FIG. 3 is a schematic end view of a roll including a wave generator transmitting a wave from a first location to a sensor at a second location.

FIG. 4 is a schematic end view of a hollow roll including a sensor.

FIG. 5 is a schematic end view of a roll with a wave starting and arriving at the same point without following the roll diameter.

FIG. 6 is a flowchart of an exemplary method of measuring thermal crown.

FIG. 7A is a schematic view of a sensor having a transmitter and a receiver according to one embodiment.

FIG. 7B is a schematic view of a sensor having a transceiver according to one embodiment.

DETAILED DESCRIPTION

Systems and processes are disclosed for directly measuring the thermal expansion of a roll, such as a metalworking roll, while the roll is hot. Thermal expansion is calculated by comparing propagation times of ultrasound waves within the roll while cold with propagation times of ultrasound waves within the roll while hot. As used herein, the term “thermal expansion” includes both positive and negative thermal expansion, such as thermal expansion and thermal contraction, where appropriate.

Measuring thermal expansion of a roll in situ (i.e., in the mill), when the rolls are hot, can enable accurate and dynamic control of the effects of thermal expansion. Specifically, it can be advantageous to control the effects of thermal crown. Obtaining an accurate measurement of the thermal expansion of the rolls in situ has many applications. For example, obtaining an accurate measurement of thermal expansion in situ when the roll is hot allows for precise adjustment of the mill setup and/or the roll cooling or heating (using actuators or otherwise). Accurate measurements of thermal expansion can enable reduction in the cool back times between product changes. Accurate measurements of thermal expansion can improve the thickness/profile of the product (e.g., a sheet of metal), as well as flatness such as edge tension in cold rolling. Accurate measurements of thermal expansion can improve the accuracy of thermal models or roll expansion and crown.

More particularly, obtaining direct measurements of the thermal expansion can be used, for example, to: (1) calculate a more accurate roll gap gauge presetting (i.e., roll gap space before the strip is introduced to the mill) so that the thickness target for the strip is achieved more quickly; (2) calculate a better roll bending presetting (i.e., roll gap space distribution across the width before the strip is introduced to the mill) so that the flatness/profile target of the strip is achieved more quickly; (3) generate better estimates of strip gauge between the stands of a multi-stand mill to improve overall speed/thickness; and (4) generate better estimates of the strip thickness profile between the stands of a multi-stand mill to improve overall flatness and/or profile.

Additionally, thermal expansion measurements can be used to quantify the variation of thermal expansion over one rotation of the roll, which can be used to assess the amount of thermal induced eccentricity. Measurements of the thermal induced eccentricity can be used to determine online when the mill is ready for rolling without inducing eccentricity-induced gauge variations after a forced outage or an emergency stop. Measurements of the thermal induced eccentricity can also be exploited by measuring each roll in the roll stack to quantify the amount of thermal eccentricity versus mechanical eccentricity when overall eccentricity is measured using standard mill sensors such as roll stand loads. These measurements, when associated with mill vibration measurements, can help to interpret vibration spectra and monitor machine condition and/or predict component failure (predictive maintenance) such as roll bearings.

Moreover, the thermal expansion measurements can be used to optimize coolant temperature and monitor the condition of roll cooling sprays for feedback control, spray optimization, thermal model optimization, and other purposes.

Obtaining accurate thermal expansion measurements can dynamically improve the accuracy of online rolling models and can be used to help make dynamic adjustments to the rolling process, as discussed above.

In some cases, the thermal expansion, crown and/or eccentricity of the work rolls is measured. In other cases, the thermal expansion, crown and/or eccentricity of intermediate and/or backup rolls is also measured.

The embodiments disclosed herein can provide for measurement of thermal expansion, crown and eccentricity with more accuracy and with less cost than other methods.

The systems and methods disclosed herein are not limited to use in rolling, but can be applied to any process or application where it is desirable to measure a dimensional change due to a thermal variation. In addition, the disclosed systems and methods can be used to calculate thermal contraction of a roll being cooled.

These illustrative examples are given to introduce the reader to the general subject matter discussed herein and are not intended to limit the scope of the disclosed concepts. The following sections describe various additional features and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative aspects, but, like the illustrative aspects, should not be used to limit the present disclosure. The elements included in the illustrations herein may be drawn not to scale.

FIG. 1 is an isometric view of a metalworking system 100 including roll 102 and a sensor bar 112. The sensor bar 112 can include one or more individual sensors 106. The roll 102 has a longitudinal axis 104 extending longitudinally through the center of the roll 102. The longitudinal axis 104 is also known as the rotational axis. The roll 102 has an exterior surface 114.

Each sensor 106 can include one or more individual devices capable of transmitting and/or receiving ultrasound waves 108. In some embodiments, a sensor 106 can be an ultrasound sensor, a phased array sensor, a shock generator, a piezoelectric transducer, a device for electromagnetic induction/measurement of mechanical waves (e.g., an electromagnetic acoustic transducer (EMAT)), a laser, or another device suitable for generating and/or measuring a mechanical wave. Each sensor 106 can include one or more transducers. In some cases, sensor 106 is an ultrasonic sensor that operates at relatively low frequencies, such as between approximately 0.5 and 10 MHz. In one non-limiting embodiment, the sensor 106 is a piezoelectric 0.5 MHz 1 inch diameter ultrasound sensor and in another is a piezoelectric 10 MHz 0.5 inch diameter ultrasound sensor.

While the present disclosure often refers to ultrasound waves 108, other mechanical waves capable of propagating through the roll 102 can be used instead.

As depicted in FIGS. 2-5, the mechanical wave 108 shown is also an indication of the wave path that the mechanical wave 108 travels.

As shown in the embodiment of FIG. 1, one or more sensors 106 are positioned at one or more fixed locations, longitudinally spaced along the width of the roll 102. Each sensor takes measurements at the sensor's 106 respective location, as described in further detail below.

In alternate embodiments, one or more sensors 106 traverse longitudinally along the width 118 of the roll 102 such that measurements are taken at a plurality of locations longitudinally spaced along the width 118 of the roll 102, as described in further detail below.

Regardless of the type of measurement system 214 used, the system 100 can use information obtained from the one or more sensors 106, the position of the sensors relative to the roll width 118, and the angular position of the sensors relative to the roll 102 to construct a three dimensional model of the thermal expansion, crown, and eccentricity of the roll. In some embodiments, rapid time-variable cooling could then be used to adjust the roll shape in a circumferential direction to control for eccentricity or otherwise. In other embodiments, distributed cooling along the roll width 118 can be applied to bring the effective thermal crown of the roll 102 to a target value. In other embodiments, overall cooling can be controlled to change the effective thermal expansion of the roll 102 to its target value.

FIG. 2 is a schematic end view of a system 200 including a roll 102 positioned relative to a sensor 106. The sensor 106 shown in FIG. 2 includes one or more transducers capable of transmitting and/or receiving a wave 108. The transducer or other mechanism for generating and/or transmitting the wave 108 may be part of the sensor 106, or may be separate from the sensor 106. The sensor 106 can be operatively connected to a processor 210 for performing data acquisition, data processing, and the calculations disclosed herein. The processor 210 can be operatively connected to a memory 212 for storing measurements, as disclosed below. The sensor 106, processor 210, and memory 212 can be considered components of a measurement system 214.

In some embodiments, a wave coupling 204 is positioned between a sensor 106 and the roll 102. The coupling 204 can be water, an emulsion, a gel, or any other suitable material or mechanism that acts as a medium for the wave 108 to propagate between the sensor and the surface 208 of the roll 102. If the wave coupling 204 is a water coupling, a water tank 206 can be used to supply water to the water coupling. For a single transmitter-receiver sensor, the dimension of the coupling layer (in the direction of the ultrasonic wave) is chosen such that the echoes from the roll-coupling interface do not interfere with the echoes from the rear side of the roll.

The system 100 (e.g., at least sensor 106 and processor 210) is configured to measure how long it takes a wave 108 to propagate inside the roll 102 in a direction substantially normal to the longitudinal axis 104 of the roll 102. As used herein, a direction substantially normal to the longitudinal axis 104 can be a direction following a line that falls within a plane substantially normal to the longitudinal axis 104, where the line can, but does not necessarily, intersect the longitudinal axis 104. The propagation time measurement in turn can be used as explained below to calculate the thermal expansion of the roll 102 at a particular point along the width of the roll 102. The propagation time of a wave 108 is sometimes referred to as a flight time, and refers to the time it takes for the wave 108 to propagate between a transmitter and a receiver or through a body (e.g., a roll 102). In some cases, the wave 108 undergoes one or multiple reflections inside the roll 102.

Thermal expansion of a roll 102 is determined by measuring the change in propagation time of a wave 108 when the roll 102 is at a reference temperature T_(R) (e.g., room temperature) and at the rolling temperature T_(H) (e.g., “in situ” temperature or “hot” temperature, as used herein). In some cases, the propagation time of the wave 108 is measured as the wave propagates through the roll 102, substantially normal to the longitudinal axis 104, and across the roll diameter 202 (FIG. 2).

The propagation time of a wave 108 propagating through a roll 102 depends on both the roll diameter 202 and the speed of sound C. Both the roll diameter 202 and the speed of sound c depend on roll temperature. As used herein, t_(R) is the propagation time of the wave 108 through the roll 102 when the roll 102 is at the reference temperature T_(R) and t_(H) is the propagation time of the wave 108 through the roll 102 when the roll 102 is at the in situ temperature T_(H). As used herein, t_(R) can be referred to as a “reference propagation time measurement” and t_(H) can be referred to as a “in situ propagation time measurement.” As used herein, Ø_(R) is the roll diameter 202 when the roll 102 is at reference temperature and Ø_(H) is the roll diameter 202 when the roll 102 is at the in situ temperature. For instance, a roll can be at reference temperature T_(R) when the roll is at a location remote from the rolling mill, or just after a roll change when the new roll is in the mill, but rolling has not started. In some embodiments, the in situ measurements are taken using the same sensor 106 taking the reference measurements. In alternate embodiments, the in situ measurements are taken using one or more different sensors 106 than those taking the reference measurements.

The change in propagation time Δt of the wave 108 from the reference thermal state (e.g., roll 102 at T_(R)) to the in situ state (e.g., roll 102 at T_(H)) can be correlated to the change in temperature ΔT (where ΔT=T_(H)−T_(R)). The change in propagation time Δt can be correlated to thermal expansion (i.e., the change in roll diameter ΔØ) and ultimately the diameter Ø_(H) along the hot roll, as described herein.

Equation 1, below, can be used to relate the change in roll diameter (ΔØ=Ø_(H)−Ø_(R)) due to a change in thermal state (ΔT) to the change in propagation time (Δt=t_(H)−t_(R)) of the ultrasonic wave due to the same change in thermal state (ΔT).

$\begin{matrix} {{\Delta\varnothing} = {\beta \; \frac{c}{2n}\Delta \; t\mspace{14mu} {assuming}\mspace{14mu} {{\frac{\Delta \; t}{t_{R}}\left( {1 - \frac{1}{\beta}} \right)}}{\operatorname{<<}1}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

In Equation 1, ΔØ is the change in roll diameter, c is the speed of sound at the reference temperature T_(R) (e.g., at room temperature), n is the number of echoes inside the roll 102, Δt is the change in propagation time of the wave between the reference temperature T_(R) and the in situ temperature T_(H) (i.e., Δt=t_(H)−t_(R)), and t_(R) is the propagation time at the reference temperature (T_(R)) (in some cases, room temperature). β is a material parameter that depends on α, the thermal expansion coefficient of the material of roll 102, and dc/dT, the change in sound speed with temperature, as seen in Equation 2, below.

$\begin{matrix} {\beta = \frac{1}{1 + \frac{{- d}\; {c/{dT}}}{\alpha \; c}}} & {{Equation}\mspace{14mu} 2} \end{matrix}$

The factor β can be determined once for a given roll 102 (or a set of rolls with the same or substantially the same material properties).

A change in diameter ΔØ between the reference temperature T_(R) and the in situ temperature T_(H) can be calculated. A reference propagation time measurement t_(R) of the roll diameter 202 can be made at the reference temperature T_(R) (e.g., at a location remote from the rolling mill) at any location along the width of the roll 102. The reference propagation time measurement t_(R) can be stored in memory 212. An in situ propagation time measurement t_(H) can be made at the in situ temperature T_(H) at various points along the width of the roll 102. The change in diameter ΔØ at each of these various points can be calculated according to Equation 1, above. The roll diameter at in situ temperature (hereinafter Ø_(H)) at each of these various points can be inferred by adding the calculated change in roll diameter ΔØ to a reference measurement of the roll diameter Ø_(R) at the reference temperature. The reference measurement of the roll diameter Ø_(R) can be made using known techniques. The reference measurement of the roll diameter Ø_(R) can be stored in memory 212.

The disclosed calculation using a change in propagation time Δt of a wave 108 is not limited to use in rolling applications, but can be used in any application or process where it is desirable to obtain the thermal expansion of any body.

Moreover, the principles described herein can be used to measure the thermal contraction of a roll 102 or any body according to the same principle, but with a reference temperature hotter than the in situ temperature (i.e., T_(R)>T_(H)).

Propagation times (e.g., t_(R) and t_(H)) can be measured using a single wave, an average of multiple waves propagating along a unique path, or an average of multiple waves propagating along multiple paths. For example, propagation times measured using an average of multiple waves propagating along multiple paths can be the average propagation time of waves passing through multiple diameters 202 of the roll 102, where each diameter 202 is located in the same plane normal to the roll axis 104. In other words, the multiple diameters 202 can be measured from various points along the circumference of the roll 102 in order to build an average propagation time in a particular plane. In other embodiments, the change in roll diameter ΔØ or the roll diameter Ø_(H) can be averaged.

FIG. 3 is a cross-sectional view of a system 300 including a roll 102. Travel of the wave 108 need not to follow the entire diameter 202 of the roll 102. The wave 108 can follow any chord which is meaningful for inferring a thermal expansion. The wave 108 does not need to travel in a direction normal to the longitudinal axis 104, but can take any direction. The formula of the thermal expansion can then be adapted by geometrical considerations.

Any reflection occurring inside the roll 102 can be exploited as well to calculate a thermal expansion, including reflections from an internal interface (e.g., an internal acoustic interface). The formula of the thermal expansion can then be adapted using geometrical considerations.

In some embodiments, a wave 108 is generated and measured at approximately the same location on the surface 208 of the roll 102 after the wave 108 reflects off of an inside surface 402 of the roll 102 (e.g., wave 108 reflecting off inner surface 402 in FIG. 4, as described in further detail below). In other embodiments the configuration is similar to FIG. 4 but the sensor is located in the hole 404 of the roll 102. In other embodiments, a wave 108 is generated and measured at approximately at the same location on the surface 208 of the roll 102 after the wave 108 followed some chord of the roll (e.g., the wave 108 reflecting off surface 208 of FIG. 5, as described in further detail below). In alternate embodiments, as shown in FIG. 3, a wave 108 is generated at a first location 302 by a transmitter 306 and measured at a second location 304 by a receiver 308. The first location 302 and/or the second location 304 can be on the surface 208 of the roll 102, within the roll 102, or located outside the roll. In some cases, the wave 108 undergoes one or multiple reflections before reaching the second location 304.

As seen in FIG. 3, the transmitter 306 is positioned opposite receiver 307 along a secant of the roll 102 intersecting the receiver 307. In other words, the receiver 307 can be positioned along a line collinear with a chord of the roll and intersecting the transmitter in order to measure waves that follow that chord of the roll. In other embodiments, the transmitter 306 and the receiver 307 may be positioned at any suitable locations along roll 102.

FIG. 4 is a cross-sectional view of a hollow roll 102 having a hole 404. Hole 404 need not be centered and need not be round as illustrated. The sensor 106 can measure the propagation time of a wave 108 as it travels through the roll 102 and is reflected off the inner surface 402 of the roll 102. The average diameter of the hole 404 can be calculated as the roll 102 makes a full rotation. When the hole 404 is eccentric, the average diameter can be used to determine the distance the wave 108 propagates through the roll 102.

FIG. 6 is a flowchart of a method of measuring thermal expansion and using the measured thermal expansions to make any desired adjustments according to one embodiment 500. In a processor 210, thermal expansion of a roll 102 is calculated at block 502. Block 502 includes measuring the propagation time of a wave through a roll at T_(R) at block 504 and measuring the propagation time of a wave through a roll at T_(H) at block 506. The thermal expansion data 512 can be used to update metalworking parameters at block 510. Metalworking parameters can include any setting or adjustment used in the metalworking process, including parameters for improving mill setup adjustment, optimization of cool back time, improving control of heat transfer from/to rolls, improving the thermal model (e.g., more frequent re-calibration), improving strip thickness control, improving strip profile control, improving strip flatness control, improving roll eccentricity compensation, and others.

FIG. 7A is a schematic illustration of a sensor 106 according to one embodiment. As used herein, a sensor 106 can include both a transmitter 602 and a receiver 604. In alternate embodiments, a sensor 106 can include only a transmitter 602. In alternate embodiments, a sensor 106 can include only a receiver 604. A transmitter 602 is any device capable of producing waves 108, such as those described in further detail above. A receiver 604 is any device capable of measuring a wave 108 propagating/reflecting on the receiver 604, such as those described in further detail above. In embodiments where a sensor 106 includes both a transmitter 602 and a receiver 604, the transmitter 602 and receiver 604 can be a single device or two separate devices co-located in a single housing.

FIG. 7B is a schematic illustration of a sensor 106 according to one embodiment. In this embodiment, the sensor 106 includes a transceiver 606 capable of both transmitting and receiving waves 108.

In some embodiments, waves 108 can further include, but not be limited to, longitudinal and traverse waves or surface waves (to measure surface temperature and roll circumference).

As discussed above, measuring the thermal crown of rolls has many potential applications. In one embodiment, average roll temperature (T_(Avg)) can be inferred according to Equation 3, below.

$\begin{matrix} {T_{Avg} = {{\frac{\beta}{\alpha}\frac{\Delta \; t}{t_{R}}} + {T_{R}\mspace{14mu} {assuming}\mspace{14mu} {{\frac{\Delta \; t}{t_{R}}\left( {1 - \frac{1}{\beta}} \right)}}{\operatorname{<<}1}}}} & {{Equation}\mspace{14mu} 3} \end{matrix}$

Equation 3 or other equations using the change in propagation time (Δt) of a wave are not limited to use in rolling applications, but can be used in any application or process where it is desirable to obtain the temperature of any body (e.g., T_(Avg)). Inferring the temperature of a roll 102 can help, for example, obtain a more accurate cooling model. A cooling model can be any mathematical formulation relating some parameters (e.g., parameters for actuators controlling water cooling flow, pressure distribution, or heating devices) to the temperature of the roll. From the average roll temperature (T_(Avg)) and the reference temperature (T_(R)), the thermal expansion can be inferred using the thermal expansion coefficient (α).

The average roll surface temperature can be inferred from the change in travel time of a surface wave traveling along the roll circumference in a similar way as the average roll temperature measurement.

Assuming a steady thermal state, the difference in the thermal expansion (e.g. thermal crown) measured with and without rolling load can be exploited using acoustoelasticity to calculate the stress distribution inside the roll.

The change in roll diameter ΔØ or roll temperature ΔT can be calculated using only measurements of waves 108 propagating in a roll 102. There is no need for external temperature measurement devices or additional distance-measuring devices. Accurate calculations of thermal expansion and change in temperature of a roll 102 can be made using only two measurements: t_(R) and t_(H).

Table 1 is a reference of symbols used throughout this disclosure. The meaning of each symbol is listed below for reference and shall not be limiting in nature.

TABLE 1 Symbol Meaning T_(R) Reference temperature (e.g., room temperature) T_(H) In situ temperature (e.g., hot temperature of a roll in use) ΔT Change in temperature between T_(H) and T_(R) t_(R) Propagation time of a wave through a roll at T_(R) t_(H) Propagation time of a wave through a roll at T_(H) Δt Change in propagation time of the wave between T_(R) and T_(H) ΔØ Change in roll diameter Ø_(H) Roll diameter at T_(H) Ø_(R) Roll diameter at T_(R) c Speed of sound at T_(R) n Number of echoes inside the roll α Thermal expansion coefficient of the material of the roll β A material parameter that depends on α and dc/dT $\frac{dc}{dT}$ Change in sound speed with temperature T_(Avg) Average roll temperature

All patents, publications and abstracts cited above are incorporated herein by reference in their entirety. Various embodiments have been described. These embodiments are presented only for the purpose of illustration and description and are not intended to be exhaustive or limiting to the precise forms disclosed. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art. 

What is claimed is:
 1. A system for dynamically measuring a parameter of a roll, comprising: a measurement system for calculating the parameter of the roll using an in situ propagation time and a stored reference propagation time comprising: a transmitter positioned with respect to the roll to generate a mechanical wave in the roll substantially normal to a longitudinal axis of the roll; and a receiver positioned with respect to the roll to measure a propagation time of the mechanical wave through the roll; wherein the parameter is selected from the group consisting of an average temperature of the roll at an in situ temperature of the roll and a change in diameter of the roll between a reference temperature and the in situ temperature.
 2. The system of claim 1, wherein the transmitter and the receiver are located in a single sensor longitudinally movable along a width of the roll.
 3. The system of claim 1, additionally comprising: a second transmitter spaced apart from the transmitter and positioned with respect to the roll to generate a second mechanical wave in the roll substantially normal to the longitudinal axis of the roll; and a second receiver positioned with respect to the roll to measure a second propagation time of the second mechanical wave through the roll to be used for the calculation of a second parameter of the roll using a second in situ propagation time and the reference propagation time, the second parameter being selected from the group consisting of a second average temperature of the roll at the in situ temperature of the roll and a second change in diameter of the roll between the reference temperature and the in situ temperature.
 4. The system of claim 1, wherein the measurement system is operable to perform, based on the parameter, one selected from the group consisting of: adjusting mill setup; adjusting roll cooling; adjusting roll heating; optimizing coolant temperature; monitoring a condition of roll cooling sprays for feedback control; optimizing cooling sprays; and optimizing a thermal model.
 5. The system of claim 1, wherein the mechanical wave is an ultrasound wave.
 6. The system of claim 1, wherein the roll is a hollow roll and the mechanical wave propagates between an exterior surface of the roll and an inner surface of the roll.
 7. The system of claim 1, wherein the receiver is positioned opposite the transmitter along a line collinear with a chord of the roll and intersecting the transmitter.
 8. A method for dynamically measuring thermal expansion, comprising: measuring a first propagation time of a first mechanical wave through a roll at a first temperature; measuring a second propagation time of a second mechanical wave through the roll at a second temperature; and calculating a parameter of the roll by comparing the first propagation time to the second propagation time; wherein the parameter of the roll is selected from the group consisting of: an average temperature of the roll at the second temperature; and a change in diameter of the roll between the first temperature and the second temperature.
 9. The method of claim 8, wherein the parameter is the change in diameter of the roll and calculating the parameter is performed using an equation including ${{\Delta\varnothing} = {\left( \frac{1}{1 + \frac{{- d}\; {c/{dT}}}{\alpha \; c}} \right)\frac{c}{2n}\Delta \; t}};$ wherein: ΔØ is the change in diameter; c is a speed of sound at the first temperature; dc/dT is a change in the speed of sound with temperature; α is a thermal expansion coefficient of the roll; n is a number of echoes inside the roll; and Δt is a difference between the second propagation time and the first propagation time.
 10. The method of claim 8, wherein the parameter of the roll is the average temperature of the roll at the second temperature and calculating the parameter is performed using an equation including ${T_{Avg} = {{\frac{\left( \frac{1}{1 + \frac{{- d}\; {c/{dT}}}{\alpha \; c}} \right)}{\alpha}\frac{\Delta \; t}{t_{R}}} + T_{R}}};$ wherein: T_(Avg) is the average temperature of the roll at the second temperature; c is a speed of sound at the first temperature; dc/dT is a change in the speed of sound with temperature;  α is a thermal expansion coefficient of the roll; t_(R) is the first propagation time;  T_(R) is the first temperature; and Δt is a difference between the second propagation time and the first propagation time.
 11. The method of claim 8, wherein: the parameter of the roll is the change in diameter of the roll; the second mechanical wave propagates through the roll in a first direction; and the method additionally comprises: measuring a third propagation time of a third mechanical wave through the roll at the second temperature, the third mechanical wave propagating through the roll in a second direction longitudinally spaced apart from the first direction; and calculating a second change in diameter of the roll between the first temperature and the second temperature. 